Method for production propylene from a flow containing C4 to C8 olefins

ABSTRACT

In a method for the production of propylene, a charge stream containing C 4  to C 6  olefins is evaporated, superheated, mixed with hot water vapor, the olefins vapor mixture converted on a zeolite catalyst, the reaction mixture formed thereby cooled, and then partially condensed. In order to increase the yield of propylene, the gaseous phase containing ethylene, propylene, C 4  to C 8  olefins, and additional hydrocarbons that is accumulated during the partial condensation is compressed, the gaseous and liquid phase containing propylene, ethylene, and other light hydrocarbons that exit from the compression step is separated into a gaseous phase containing propylene, ethylene, and other light hydrocarbons and a liquid phase containing C 4 + olefins, and the liquid phase is separated into a fraction containing C 4  to C 6  olefins and a fraction containing C 7 + olefins.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of PCT applicationPCT/EP2003/005903, filed 5 Jun. 2003, published 29 Jan. 2004 as WO2004/009519, and claiming the priority of German patent application10233069.7 itself filed 19 Jul. 2002.

The invention relates to a method for the production of propylene from aliquid charge stream containing C₄ to C₈ olefins that evaporates at 25to 200° C. and is superheated to 350 to 400° C., wherein the formedvapor containing the olefins is mixed with hot water vapor, the olefinsvapor mixture converted on a shape-selective, pentasil-type zeolitefixed-bed catalyst at inlet temperatures of 450 to 550° C. and pressuresof 0.5 to 3.0 bar (abs), the reaction mixture formed thereby is cooledto 100 to 200° C., and through a subsequent further cooling to 40 to<100° C. a partial condensation is carried out with formation of agaseous phase containing essentially ethylene, propylene, C₄ to C₈olefins and additional hydrocarbons and a liquid phase that isessentially comprised of water and is returned into the charge stream.

In order to be able to satisfy the increasing demand worldwide forpropylene that is accumulated to roughly 98% as a by-product duringthermal cracking (Steamcracking) and during catalytic cracking in afluid bed (Fluid Catalytic Cracking, FCC) of crude oil fractions, theexperts aim to make propylenes in a secondary manner. For this purpose,according to Cit.:HYDROCARBON ENGINEERING May 1999, p. 66/67, C₄+olefins, such as butylenes, pentenes, hexenes etc. are fragmented bymeans of a shape-selective, pentasil-type zeolite catalyst into“CH₂”-units that then recombine to propylene, ethylene, and butylene ina quasi-equilibrium distribution, whereby the conversion rate is approx.83 percent by weight (42 percent by weight propylene, 31 percent byweight butylene, and 10 percent by weight ethylene) based on the olefinsin the charge steam. In case of a return of butylene into the processcycle, even a yield of 60 percent by weight of propylene and 15 percentby weight of ethylene results. For the production of propylene, thecharge stream containing C₄+ hydrocarbons that is mixed with returnedbutylene is evaporated after passing through a saturator, after additionof recycle water vapor, in a heat exchanger at temperatures of 20 to100° C., subsequently superheated in a heat exchanger and in an oven toa temperature of 100 to 500° C., and then supplied to a reaction vesselfilled with a ZSM-5-type shape-selective zeolite fixed-bed catalyst,preferably of the Claus-type. The gas stream exiting from the reactionvessel is cooled in a heat exchanger to a temperature of 100 to 200° C.,compressed to a pressure of 2 to 6 bar (abs), afterwards charged to thecondensation side of an evaporator/condenser and the hydrocarbons thatremained in the gaseous phase directed to a gasoline/olefin-splitter,while the condensate that is primarily comprised of water is evaporatedin the evaporator/condenser after a lowering of the pressure andsupplied to the saturator as recycle water vapor. The gaseous phase isseparated into a fraction containing C₃ hydrocarbons and gasoline in thesplitter.

It is the object of the present invention to increase the yield ofpropylene by means of the method described in the beginning, withoutincreasing the technical requirements.

The object is solved by compressing the gaseous phase that containsethylene, propylene, C₄ to C₈ olefins and additional hydrocarbons formedin a partial condensation carried out by means of a quenching step to apressure of 20 to 30 bar (abs), separating the gaseous and liquid phaseexiting from the compression step into a gaseous phase containingessentially propylene, ethylene and other light hydrocarbons and aliquid phase containing C₄+ olefins, and separating the liquid phaseinto a fraction containing C₄ to C₆ olefins and a fraction containingC₇+ olefins.

A preferable embodiment of the method is to be seen therein, that thewater stream accumulated as condensate in the quenching step isre-evaporated, then heated to a temperature of 600 to 800° C., andreturned to the charge stream that contains vaporous hydrocarbons. Bythis measure, the charge stream is heated to the inlet temperature of450 to 550° C. required for the reaction step, so that it can be donewithout the heating of the charge stream containing the hydrocarbons bymeans of a special oven.

According to another feature according to the invention, the majority ofthe produced C₄ to C₆ olefins is returned to the charge streamcontaining vaporous hydrocarbons, in order to further increase the yieldof propylene.

It is further advantageous, to evaporate the water that accumulates inthe compression step, to then heat the vapor to a temperature of 600 to800° C. and re-add it to the charge stream containing vaporoushydrocarbons.

The invention is exemplary illustrated through a process diagram in thedrawing and is further explained hereinafter.

The charge stream containing C₄ to C₈ olefins is charged via line (1) toan evaporator (2), in which it is heated to a temperature of 100° C. andevaporated at a pressure of 6.5 bar (abs). The charge stream flows vialine (3) into a superheater (4), in which it is superheated to atemperature of 350° C. For further superheating of the charge stream toa temperature of 500° C., the charge stream is mixed with 700° C. hotwater vapor that is supplied via line (5) and fed into line (6) by meansof the mixing device (7). The gas stream that has a temperature of 500°C. is directed into the reactor (19) via line (8). In an endothermicadiabatic reaction on a zeolite fixed-bed catalyst, the majority of theC₄ to C₈ olefins is converted into C₃ to C₆ olefins with propylene asthe main component. The reaction mixture that has a reaction temperatureof 460° C. and is withdrawn via line (10) is cooled in the heatexchanger (11) to a temperature of 200° C. Subsequently, the reactionmixture is charged via line (12) to a quenching column (13), in which acooling to 60° C. occurs at the head of the column. The gaseous phaseaccumulating in the quenching column (13) is supplied via line (14) to acompressor (15), compressed to a pressure of 27 bar (abs), and cooled toa temperature of 60° C. The gaseous and liquid phase containinghydrocarbons that are formed in the compressor (15) are fed via lines(16) and (17), respectively, into a distillation column (18), throughthe head of which via line (19) the gaseous phase that has a temperatureof 20° C. and contains propylene, ethylene, and other lighthydrocarbons, and from the bottom of which via line (20) a liquid phasethat contains essentially C₄+ olefins are withdrawn. The head producthas a propylene content of about 75 percent by weight. The bottomproduct is supplied to a distillation column (21), in which it isseparated into a gaseous phase containing C₄ to C₆ olefins with atemperature of 50° C. and a liquid phase containing C₇+ olefins. Thegaseous phase that exits from the distillation column (21) via line (22)is charged to about 65% via line (26) to a superheater (27) andafterwards is returned via line (28) to the heat exchanger (4) for thesuperheating of the vaporous charge stream, while the remainder of thegaseous phase is discharged via line (29). The liquid phase that isleaving the distillation column (21) is recovered from the process vialine (23). The water stream exiting from the quenching column (13) isdirected via line (24) into the evaporator (25), and the water vaporformed therein that after superheating has a temperature of 700° C. isadded via line (5) to the charge stream containing vaporous hydrocarbonsand having a temperature of 350° C., before the charge stream flows intothe mixing device (7). The water that is formed in the compressor (15)is fed via line (30) into the water stream that is withdrawn from thequenching column (13) through line (24).

1. A method for producing a hydrocarbon mixture rich in propylene, consisting essentially of propylene, ethylene and other light hydrocarbons from a liquid charge stream containing C₄ to C₈ olefins, which comprises the steps of: (a) charging the liquid charge stream containing C₄ to C₈ hydrocarbons into an evaporator at a temperature of 25 to 200° C. to evaporate the liquid stream; (b) superheating the evaporated liquid stream at a temperature of 350 to 400° C. followed by an additional superheating of the evaporated liquid stream to 450 to 550° C. using hot water vapor; to form an olefin—water vapor mixture; (c) adiabatically reacting the olefin—water vapor mixture, superheated according to step (b), over a shape-selective, pentasil zeolite fixed-bed catalyst to convert a majority of the C₄ to C₈ olefins in the olefin—water vapor mixture to a mixture of C₃ to C₆ olefins rich in propylene; (d) following step (c), cooling the olefin—water vapor mixture to a temperature of 100 to 200° C.; (e) quenching the olefin—water vapor mixture cooled according to step (d), to a temperature of 40 to <100° C., to partially condense the olefin—water vapor mixture; thereby obtaining a gaseous hydrocarbon phase consisting essentially of ethylene, propylene, C₄ to C₈ olefins and additional hydrocarbons and, a liquid phase consisting essentially of water that is returned to the evaporated liquid stream during step (b); (f) compressing the gaseous hydrocarbon phase obtained according to step (e) at a pressure of 20 to 30 bar absolute to remove accumulated water from the gaseous hydrocarbon phase to obtain a mixture of gaseous and liquid hydrocarbon phases; (g) separating the mixture of gaseous and liquid hydrocarbon phases into a gaseous hydrocarbon phase, rich in propylene, consisting essentially of propylene, ethylene, and other light hydrocarbons, and recovering said gaseous hydrocarbon phase and a liquid hydrocarbon phase containing C₄+ olefins; and (h) separating the liquid hydrocarbon phase into a fraction containing C₄ to C₆ olefins and a fraction containing C₇+ olefins.
 2. The method defined in claim 1, wherein according to step (e) the liquid phase consisting essentially of water accumulated as a condensate during the quenching is reevaporated, then heated to a temperature of 600 to 800° C., and then returned to the liquid evaporated stream during step (b).
 3. The method defined in claim 1, wherein following step (h) the majority of the generated C₄ to C₆olefins is returned to the liquid charge stream according to step (a).
 4. The method defined in claim 1, wherein according to step (f) the accumulated water, separated from the gaseous and liquid hydrocarbon phases is evaporated, then heated to a temperature of 600 to 800° C., and returned to the liquid evaporated stream during step (b).
 5. The method defined in claim 1 wherein according to step (g) the gaseous hydrocarbon phase, rich in propylene consists essentially of 75% propylene. 